Every tenth lever-press of three morphine-dependent rhesus monkeys was reinforced with food. A red light, initially a neutral stimulus, was presented every third or fourth session for 5 min before and 5 min after an intravenous injection of nalorphine, a morphine antagonist that produces an immediate abstinence syndrome in morphine-dependent monkeys. After several pairings, conditioned suppression of lever pressing, heart-rate decrease, vomiting, and excessive salivation were observed during the red-light period before nalorphine injection. No conditioned electrocardiogram, respiration or temperature changes occurred. After 10 red light-nalorphine pairings, morphine administration was completely discontinued and monkeys were then tested monthly for persistence of the conditioned responses. The red light paired with saline injection continued to suppress lever pressing and to produce heart-rate decreases after 60 to 120 days of complete abstinence from morphine. Subsequently, daily presentations of the red light-saline injection complex rapidly extinguished these conditioned responses. Nevertheless, they could be rapidly reinstated by additional nalorphine injections.